1. Field of the Invention
This invention relates to a modified, thermosetting urea-formaldehyde resin composition useful for preparing a binder formulation, such as for making fiber mats, particularly glass fiber mats, to a process of preparing the fiber mats using the binder formulation and to the fiber mats prepared using the modified urea-formaldehyde resin composition in a binder formulation. In particular, the invention relates to a thermosetting urea-formaldehyde (UF) resin modified by the addition of a rheological-enhancing amount of a thickening agent.
2. Description of Related Art
Glass fiber mats, fiber mats made from synthetic fibers and mats made from fiber blends find wide application in the building materials industry, for example, as insulation, as a substrate for making composite flooring, as a substrate for making roofing shingles, as a substrate for making siding (replacing similar sheets traditionally made using wood, cellulose or asbestos fibers), as substrate for printed circuit boards or battery separators, as filter stock, as tape stock, as reinforcement scrim in cementitious and non-cementitious coatings for masonry and as facing material for laminated articles such as foam composites and gypsum boards.
Such fiber mats are made in a range of thickness and densities, by entangling, often short, staple fibers, long continuous fibers and mixtures thereof, and are generally referred to as non-woven mats.
Some non-woven fiber mats, and especially relatively thin non-woven glass fiber mats, usually are made commercially by a wet-laid process, which is carried out on what can be viewed as modified papermaking machinery. Descriptions of the wet-laid process may be found in a number of U.S. patents, including U.S. Pat. Nos. 2,906,660; 3,012,929; 3,050,427; 3,103,461; 3,228,825; 3,760,458; 3,766,003; 3,838,995; 3,905,067; 4,112,174; 4,681,802 and 4,810,576.
In general, the wet-laid process for making fiber mats comprises first forming an aqueous slurry of short-length fibers, usually glass fibers (referred to in the art as “white water”) under agitation in a mixing tank, then feeding the slurry onto a moving screen on which the fibers enmesh themselves into a freshly prepared wet fiber mat, while excess water is separated from the mat of fibers.
Machines such as wire cylinders, Fourdrinier machines, Stevens Former, Roto Former, Inver Former, Venti Former machines and the like can be used to form the wet-laid mat. In such equipment, a head box deposits the slurry onto a moving wire screen. Suction or vacuum removes the water resulting in the wet-laid mat.
Unlike natural fibers such as cellulose or asbestos, glass fibers and some synthetic fibers generally do not disperse well in water. To overcome this problem, it has been the industry practice to provide suspending aids for the fibers. Such suspending aids or dispersants usually are materials that increase the viscosity of the aqueous medium. Suitable dispersants conventionally employed in the art include polyacrylamides, hydroxyethyl cellulose (HEC), carboxymethyl cellulose (CMC), ethoxylated amines and amine oxides. Other additives such as surfactants, lubricants and defoamers also conventionally have been added to the white water. Such agents, for example, further aid the wettability and dispersion of the fibers. Experience has shown that such additives also often influence the strength of the wet fiber mat.
The fiber slurry deposited on the moving screen or cylinder is processed into a sheet-like fiber mat by the removal of water, usually by suction and/or vacuum devices, and is followed by the application of an adhesive binder formulation to the mat. In the manufacture of fiber mats, a high degree of flexibility and tear strength is desired in the finished mat in addition to primary dry tensile and wet tensile properties. An adhesive binder formulation is therefore used to hold the fiber mat together. The binder formulation is impregnated directly into the fibrous mat and set or cured immediately thereafter (usually by the application of heat by passing the mat through a drying oven) to provide the desired mat integrity.
The binder formulation is applied to the mat by soaking the mat in an excess of binder solution or suspension, or more usually by impregnating the mat with the adhesive binder formulation by means of a binder applicator, for example, by a roller coater, curtain coater, dip and squeeze applicator, or spray coater to name a few. One of the primary binder applicators for conventional fiber mat machines has been the falling film curtain coater. Suction devices often are also utilized for further removal of water and excess binder and to ensure a thorough application of binder through the full thickness of the fiber mat.
A widely used binder formulation for making fiber mats, especially glass fiber mats, is based on a thermosetting urea-formaldehyde (UF) resin, which often is fortified with an emulsion polymer. UF resins have commonly been employed because they are relatively inexpensive In addition to mat strength properties which the binder formulation imparts to the ultimately cured mat, the binder also functions to improve the strength of the uncured, wet-laid mat as it is transported from its initial formation into and through the curing oven. Such incipient pre-cured strength is needed to avoid process delays and shutdowns caused by breaks in the endless mat.
Thus incorporated adhesive binder formulation is dried and thermally cured, typically in an oven at elevated temperatures. Generally, a temperature in the range of about 200 to 400° C. is used during the heating step. The necessary heat energy can be supplied in any manner known in the art including direct-fired ovens, convection ovens, or by radio frequency (RF) or dielectric heating. Normally, heat treatment alone will effect curing of the binder. Catalytic-assisted curing, such as is accomplished with addition of an acid catalyst (for example ammonium chloride or p-toluene sulfonic acid) to the binder composition is an optional, alternative. The resulting cured mat is normally wound into rolls, packaged and shipped to a point of use.
Because fiber mats made with an adhesive binder formulation consisting predominantly of a UF resin often are brittle, or because the strength properties of the mats may deteriorate appreciably subsequent to their preparation, especially when the mats are subjected to wet conditions, UP resin-based adhesive binder formulations have often been modified by fortifying the UF resin with a large amount of latex (emulsion) polymer, usually a polyvinyl acetate, vinyl acrylic or styrene-butadiene polymer. Certain latexes have been shown to provide increased tensile strength and tear strength in the fiber mat. The use of styrene-butadiene latex-fortified, urea-formaldehyde resin compositions in preparing an adhesive binder formulation for making glass fiber mats is disclosed, for example, in U.S. Pat. Nos. 4,258,098; 4,560,612 and 4,917,764.
U.S. Pat. No. 4,917,764 highlights another problem sometimes encountered during the manufacture of fiber mats, especially when using the falling film curtain coater adhesive binder application technique. In particular, conventional UF resin-based adhesive binder formulations are sometimes deficient in providing an even deposition of the adhesive binder formulation on the mat. This uneven application of the adhesive binder formulation to the mat contributes to non-uniform mat properties.
Applicant now has determined that this problem is related to the rheological behavior of such UF resin-based adhesive binder formulations. In particular, applicant has discovered that by modifying a thermosetting UF resin composition with a small amount of a thickening agent, the rheological characteristic of an adhesive binder formulation made using the thermosetting UF resin composition is significantly improved, allowing the adhesive binder formulation to be applied evenly at high mat manufacturing speeds even using a curtain coater.